Technologies by which optical head apparatuses that record or read information onto or out of optical disks are made into thin films have been proposed by, for example, the 46th academic lecture meeting 2p-L-15 of the Applied Physics Association of Japan. FIG. 1 shows the construction of this conventional thin film optical head apparatus.
In FIG. 1, the reference numeral 133 is a semiconductor laser, 132 is a waveguiding layer, 135 is a grating beam splitter (GBS), 136 is a focusing grating coupler (FGC), 11 is an optical disk, 140A, 140B and 141A, 141B are light detectors. The waveguiding layer 132 is formed over a substrate 131 to sandwich a dielectric layer with a low refractive index therebetween. Laser light emitted by the semiconductor laser 133 spreads while passing through the waveguiding layer 132 to become waveguided light 134 in a TE mode. The waveguided light 134 is converted to parallel light rays by the GBS 135 which is formed on the waveguiding layer 132, and a part of the parallel light becomes radiation mode light 137. The radiation mode light 137 is focused onto a focal point FC and is reflected by the reflecting surface of the optical disk 11 located at the focal point FC to return to the FGC 136 where it is converted to waveguided light again. The waveguided light is split by the GBS 135 into waveguided lights 138 and 139, which are then focused onto the light detectors 140A, 140B and 141A, 141B.
The reflecting surface of the optical disk 11 is provided with guide grooves running along the rotation direction 12 of the disk and arranged periodically in a radial direction, to thereby diffract the reflected light in the radial direction of the disk. Because the interference of the diffracted light causes tracking error (TE) appearing as an unevenness in the amount of the reflected light 137 in the radial direction of the disk, a TE signal can be obtained by measuring the amounts of waveguided lights 138 and 139 and taking the difference therebetween (so-called push-pull detection). By splitting the waveguided light by means of the GBS 135, the defocus of the optical disk reflecting surface is represented by the difference in the light amount distribution on the light detectors 140A and 140B or 141A and 141B, based on the same principle as the focus error (FE) detector by means of a knife edge. Consequently the TE signal is obtained by taking the difference between the summation signal of light detectors 140A and 140B and the summation signal of 141A and 141B by a differential amplifier 144, and the FE signal is obtained by taking the difference between the summation signal of light detectors 140A and 141A and the summation signal of 140B and 141B by a differential amplifier 143. On the other hand, the summation signal of the light detectors 140A, 140B, 141A and 141B is obtained by means of a summing amplifier 142 and is used as a reproduced signal.
However, optical head apparatuses of such a construction as described above have the following problems.
First, a semiconductor laser causes fluctuation in the wavelength depending on the temperature and the magnitude of the output power, so that the diffraction angle of light by the GBS 135 changes, causing the waveguided light 134 to be incident on the FGC 136 in a state that deviates from parallel light. Therefore, aberration (astigmatism in particular) is generated in the radiation light 137 due to unparallelism and the difference in the optical path. Also, because the diffraction angle of the radiation light 137 from the FGC 136 changes, the angle of incidence onto the optical disk 11 changes, and coma arises by the action of a transparent substrate which covers the reflecting surface of the optical disk. These aberrations cause the focusing characteristics of the light distribution on the reflecting surface to deteriorate and the reproduction performance (or recording performance) to decrease. Moreover, the light focusing points 138F and 139F of the two waveguided lights 138 and 139 which are focused on the light detectors 140A, 140B and 141A, 141B are dislocated as indicated by the arrows, because the diffraction angle of light on the GBS 135 changes in accordance with the wavelength fluctuation. This dislocation of the spot on the light detector disturbs the FE control signal resulting in a defocus on the reflecting surface of the optical disk, thereby to further lower the reproduction performance (or recording performance).
Second, if the thickness and the refractive index of the waveguiding layer deviate from the design values, the equivalent refractive index of the waveguiding layer also deviates, and the diffraction angle of light due to the GBS 135 deviates, causing the waveguided light 134 to enter the FGC 136 in a state that deviates from parallel light. The diffraction angle of the radiation light 137 from the FGC 136 also deviates. Similar to the wavelength fluctuation, aberration is generated in the radiation light 137, and focusing characteristics of the light amount distribution on the reflecting surface deteriorate, so that the reproduction (or recording) performance is lowered.
Third, in FIG. 2 which shows the intensity distribution of the light 145 emitted from the FGC, the output coupling efficiency .eta..sub.0 of the FGC 136 is represented as .eta..sub.0 =(Distribution ratio).times.(Light intensity of P1)/(Light intensity of P1+P2). P2 is the amount of radiation light when assuming that the grating of the FGC is extended. The distribution ratio in the case of 2-beam coupling is generally 0.5. When the radiation loss factor of FGC is increased, the intensity distribution shown in FIG. 2(a) is turned into the intensity distribution shown in FIG. 2(b) while (light amount of P1+P2) remaining constant. At this time, the output coupling efficiency .eta..sub.0 increases because the light amount of P1 increases. However, the intensity distribution of FIG. 2(b) is uneven in comparison to the intensity distribution of FIG. 2(a), resulting in a decrease in NA and substantial deterioration of the focusing characteristics at the focal point. That is, the output coupling efficiency and the light focusing characteristics are in an inverse relation.
Fourth, in FIG. 3 which shows the intensity distributions of the light 145 emitted from the FGC and of the returning light 146 which returns from the optical disk reflecting surface, the intensity distribution C of the returning light becomes symmetric with the intensity distribution A of the emitted light, by reflecting on the reflecting surface at the focal point. While the input coupling efficiency .eta..sub.1 (coupling efficiency at which the returning light 146 is converted to waveguided light 147) generally increases when the output distribution and the input distribution of the grating coupler are orthomorphic, the input coupling efficiency .eta..sub.1 is small because the intensity distribution of the returning light of C is not orthomorphic with the intensity distribution of the emitted light of A. In addition, although waveguided light which passes the grating coupler without being radiated and waveguided light with a vector opposite thereto are needed as a pumping light to increase the coupling efficiency at which the returning light is added to the waveguided light, the waveguided light of an opposite vector does not exist at the actual input coupling. Therefore, a requirement for an increase in the input coupling efficiency is that the waveguided light of the opposite vector is of a small amount, namely the amount of waveguided light which passes the grating coupler without being radiated is small (that is, the amount of P2 is small). Although P2 must be increased to obtain higher focusing characteristics as previously described, an improvement of the input coupling efficiency and an improvement in the focusing characteristics are in an inverse relation and are incompatible, as an increase in P2 results in a decrease in the input coupling efficiency. Therefore, by the principle, the input coupling efficiency of the returning light becomes low, and the light amounts detected by the light detectors 140A, 140B and 141A, 141B are low, resulting in poor qualities (S/N ratio) of the control signals and the reproduced signals.
Fifth, the diffraction efficiency of the transmitted light of the GBS 135 is low when the efficiency of the transmitted wave is high, and the efficiency of the transmitted wave is low when the efficiency of the diffraction wave is high. Therefore if the amount of light of the transmitted wave directed toward the FGC 136 is increased, the light amount of the diffracted waves directed toward the light detectors 140A, 140B and 141A, 141B becomes small, and if the amount of light of the diffracted waves directed toward the light detectors are increased, the amount of transmitted wave directed toward the FGC becomes small. In other words, the transmission efficiency of the light to the optical disk reflecting surface and the transmission efficiency thereof to the light detector cannot be improved at the same time. Also the transmitted wave of the returning light returns to the semiconductor laser 133 and the feedback coupling therewith causes a disturbance in the oscillation of laser.
Sixth, while the focusing characteristics (Strehl's Definition) is proportional to (Aperture of FGC)/(Focal distance of FGC).sup.2 because the focal distance cannot be decreased due to the vertical run-out of the optical disk and other causes, a sufficiently large aperture of the FGC (e.g., 4 mm.times.4 mm) is needed to secure a sufficient level of focusing characteristics. The aperture of the FGC here means the area of the aperture for the radiation light on the FGC, and the aperture area is determined by the distance between the semiconductor laser and the FGC and by the spreading angle of the waveguided light 134 emitted from the semiconductor laser 133. As the spreading angle is as small as 10 to 20 degrees in the case of waveguided light of a TE mode, a relatively large distance (2-3 cm) between the semiconductor laser and the FGC is require for a sufficient FGC area (aperture area), which causes difficulty in making a compact apparatus.
Seventh, the grating of the FGC, which depicts a complicated curve (biquadratic equation), makes its processing difficult. Although some methods such as electron beam lithography are suited to a processing with complicated curves, the specified processing accuracy is guaranteed only for a small drawing area of 1 mm.times.1 mm, and, therefore, they are not suitable for the processing of gratings with a sufficiently large area.